Warn on invalid UTF-8 in AGENTS.md files

[etraut-openai]

Fixes #23223.

Why

Malformed AGENTS instructions should not fail silently. The reported issue had invalid UTF-8 in a global AGENTS.md; before this change, Codex treated that decode failure like a missing file, so the personal instructions disappeared without a user-visible explanation and the rollout had no # AGENTS.md instructions block.

Project-level AGENTS files already used lossy decoding, so their instructions still appeared, but invalid bytes were replaced without telling the user. Global and project AGENTS files should behave consistently: keep usable instruction text when possible, and surface a diagnostic when bytes had to be replaced.

What changed

Global AGENTS.override.md and AGENTS.md loading now reads bytes and decodes with replacement characters on invalid UTF-8, matching project-level AGENTS behavior. Both global and project AGENTS loading now emit a startup warning when invalid UTF-8 is found, and both keep the instruction text with invalid byte sequences replaced.

Missing files, non-file candidates, empty files, and the existing AGENTS.override.md before AGENTS.md precedence keep their current behavior.

How users see it

The warnings flow through the existing startup warning surface. App-server clients receive config-time startup warnings as configWarning notifications during initialization, and thread startup emits startup warnings as thread-scoped warning notifications.

Global AGENTS invalid UTF-8 warnings can appear on both surfaces. Project-level AGENTS invalid UTF-8 warnings are discovered while building thread instructions, so they appear as thread-scoped warning notifications. Clients that render warning notifications in the conversation surface show the message as a visible diagnostic instead of silently hiding or altering instructions.

[bolinfest]

It's slightly better to just std::fs::read() and then check to see whether err is ENOENT (and also EISDIR to be extra conservative?) rather than the separate is_file() check to avoid TOCTOU, though I concede the risk is minor.

[bolinfest]

If warn_invalid_utf8() returned Option<String>, returning Some when std::str::from_utf8() succeeds, you would only do the pass once in the common case:

Suggested change

	warn_invalid_utf8(&path, &data, "Global", startup_warnings);
	let contents = String::from_utf8_lossy(&data);
	let contents = warn_invalid_utf8(&path, &data, "Global", startup_warnings).unwrap_or_else(|| String::from_utf8_lossy(&data));